1. Field of the Invention
This invention relates to a torque detecting apparatus for detecting steering torque applied on a steering shaft for steering operation, and an electric power steering apparatus for driving and controlling a steering force assisting motor based on the steering torque detected by the torque detecting apparatus.
2. Description of the Prior Art
An electric power steering apparatus, which drives a steering force assisting motor based on detection result of steering torque applied on a steering wheel for steering operation and transmits turning effort from the motor to a steering mechanism to assist steering, can control assisting force characteristics according to a running state, such as vehicle speed and frequency of steering, more easily than a hydraulic power steering, which uses a hydraulic actuator as a source of assisting steering force. Such an advantage has recently invited an inclination to allow wider application of the electric power steering apparatus.
Such an electric power steering apparatus is under the necessity of detecting steering torque applied on the steering wheel. The detection is generally accomplished by detecting rotational angles at two positions of a steering shaft which are distant in the axial direction of the steering shaft, which connects the steering wheel and the steering mechanism. Steering torque can be calculated based on a difference between the two rotational angles detected at said two positions, since the difference corresponds to a torsion of the steering shaft generated in the course of steering operation.
The steering shaft is constructed by connecting an input shaft on the steering wheel side and an output shaft on the steering mechanism side through a torsion bar of a small diameter. Such a construction allows a large rotational angle difference to arise according to the torsion of the torsion bar between the input shaft and the output shaft, and accuracy of calculating steering torque can be increased by detecting rotational angles of the two shafts respectively.
The present applicant has proposed, in Unexamined Japanese Patent Publication No. 2000-352502, a torque detecting apparatus in an electric power steering apparatus which can preferably be used for detecting steering torque. Said torque detecting apparatus has sensor units respectively provided at connection parts of the input shaft and of the output shaft. The sensor units comprises magnetic targets, which are provided in peripheral direction of an object rotational axis and tilted approximately equally against the axial direction of the steering shaft, and magnetometric sensors (MR sensors), which are provided on the exterior of the magnetic targets opposing the same and generates output which changes as each target passes by.
In this construction, the magnetometric sensor in the sensor units outputs voltage, which changes, as each target passes by, lineally with a slope corresponding to the tilt of the targets set in peripheral direction of the input shaft and the output shaft. The rotational angles of the input shaft and the output shaft can be detected in non-contact mode based on the output from each corresponding magnetometric sensor. The steering torque applied on the input shaft by operating the steering wheel is thus calculated based on the difference between the rotational angles of the two shafts, which is obtained as an output difference between the magnetometric sensors corresponding to the input shaft and the output shaft.
For controlling the steering force assisting motor in the electric power steering apparatus, the magnitude of the steering torque applied on the steering wheel needs to be detected as well as the direction of the steering torque, namely, whether the steering is performed to the left or to the right. The torque detecting apparatus constructed as described above judges the direction of the steering torque by comparing the rotational angle of the input shaft and that of the output shaft. The electric power steering judges that the steering torque is applied in the direction in which the input shaft precedes the output shaft.
In addition to the proper input torque (steering torque) applied from the input shaft on the steering wheel side according to the steering operation, reaction force from road surface applied on a wheel for steering acts on the steering shaft of a running vehicle as an input torque (called xe2x80x9cinverse input torquexe2x80x9d in the following) from the output shaft on the steering mechanism side. For example, at straight driving without turning the steering wheel, when the vehicle runs on rough road surface and large inverse input torque is applied, the inverse input torque may be detected by the torque detecting apparatus constructed as described above.
Inverse input torque is applied such that the rotation of the output shaft side precedes that of the input shaft side, while the proper steering torque is applied such that the rotation of the input shaft side precedes that of the output shaft side. Consequently, in judging the torque direction based on the detecting result by the torque detecting apparatus, the inverse input torque is judged to be steering torque applied in the direction, in which the wheel for steering is energized by the action of the reaction force from the road surface. When the steering force assisting motor is driven according to such a torque detection, driving force of the motor is applied on the steering mechanism in the same direction as the inverse input torque, and the driver who is at the wheel feels such that the primary inverse input torque is promoted. The conventional art thus has a problem of causing a deterioration of a steering feeling.
The present invention has been made to solve the problematic point mentioned above.
One object of the present invention is to provide a torque detecting apparatus for detecting steering torque applied on the steering shaft, which connects the steering wheel and the steering mechanism, from the steering wheel side, and inverse input torque applied on the steering shaft from the steering mechanism side, separately.
Another object of the invention is to provide an electric power steering apparatus which can prevent the deterioration of steering feeling when the inverse input torque acts, by driving the steering force assisting motor in a direction to cancel the inverse input torque, or by not driving the steering force assisting motor.
A torque detecting apparatus according to the present invention judges that inverse input torque is applied on a steering shaft which connects the steering wheel and the steering mechanism, when a predetermined interrelation exists among vehicle speed and two angular velocities which are calculated at two positions distant in the axial direction of the steering shaft. The torque detecting apparatus attempts to prevent inappropriate controlling operation by outputting torque of assistance force calculated in a predetermined way as follows. When the inverse input torque acts, the torque of assistance force is calculated by inverting the direction of the detected torque or by inverting the direction of torque obtained by multiplying the detected torque, or fixed zero.
An electric power steering apparatus according to the present invention attempts to prevent the deterioration of steering feeling caused by the action of the inverse input torque. The electric power steering apparatus detects the proper steering torque and the inverse input torque separately in the same way as the torque detecting apparatus, and, when the inverse input torque is detected, drives the steering force assisting motor in the opposite direction to the detected torque direction according to the torque of assistance force, which is output by the torque detecting apparatus, or does not drive the motor.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed description and drawing.